1. Field of the Invention
The present invention relates to a wavelength division multiplex optical amplification transmission system for relaying and transmitting a plurality of optical signals having different wavelengths by use of an optical amplifier, and to an optical amplifier used in this wavelength division multiplex optical amplification transmission system.
2. Description of the Related Art
One communication system using optical signals is a system in which a plurality of optical signals having different wavelengths are, after being multiplexed, transmitted and relayed via an optical amplifier (which will hereinafter be referred to as a wavelength division multiplex optical amplification transmission system).
When constructing the wavelength division multiplex optical amplification transmission system, a lightwave receiving terminal station is so constructed as to deal with, as communication information, only the optical signal levels which fall within a predetermined range (the lightwave receiving terminal station can not be constructed so that signals having arbitrary levels are recognized as communication information). Therefore, a condition that the level of the optical signal must fall within the predetermined range is imposed on the optical signal (outputted from the transmission line) inputted to the lightwave receiving terminal station. Then, a level of the optical signal outputted from the transmission line is determined by a level of the optical signal when inputted to the transmission line and a gain of the transmission line with respect to this optical signal. It is therefore desirable that the transmission line used in the wavelength division multiplex optical amplification transmission system be after all the one in which the gain does not depend on the wavelength.
An optical amplifier represented by an erbium-doped fiber amplifier (EDFA) and used for constituting the transmission line, however, as shown in FIG. 8, has a gain wavelength characteristic in which the gain varies corresponding to the wavelength. Accordingly, when the transmission line is constructed of only the optical amplifiers and optical fibers, it follows that this transmission line exhibits a gain wavelength characteristic in which the gain varies corresponding to the wavelength. Further, the gain wavelength characteristic is that a gain dependency on the wavelength becomes higher with a greater number of optical amplifiers provided in the transmission line
Hence, the wavelength division multiplex optical amplification transmission system including the transmission line constructed of only the optical amplifiers and the optical fibers, presents problems wherein the number of optical signals that can be multiplexed is small (a signal wavelength band is narrow), and a transmission distance can not be increased without narrowing the signal wavelength band.
In order to obviate such problems arising because of the transmission line gain depending upon the wavelength, in the wavelength division multiplex optical amplification transmission system, the gain of the transmission line (the optical amplifiers) becomes independent on the wavelength (which is called gain equalization) by inserting a gain equalizer defined as a kind of the optical filter into the transmission line. That is, it has been practiced that the gain of the transmission line is fixed by inserting into the transmission line the gain equalizer exhibiting a loss wavelength characteristic assuming the same configuration as the gain wavelength characteristic possessed by the transmission line.
A characteristic of which the gain equalizer is demanded and a method of constructing (designing) the prior art gain equalizer will hereinafter be specifically explained.
FIG. 9 schematically shows a construction of the wavelength division multiplex optical amplification transmission system involves the use of the gain equalizer. As shown in FIG. 9, the gain is equalized by providing a single gain equalizer 32 in every section containing normally a plurality (generally, 5-10 units) of optical amplifiers 31. More specifically, each gain equalizer is designed and manufactured to have a loss wavelength characteristic including a wavelength area in which a configuration is coincident (relative values are equal) with a gain wavelength characteristic of a structure (hereinafter referred to as an equalization target module) obtained by connecting the optical amplifier 31 to the optical fiber through no intermediary of the gain equalizer 32, which are contained in the corresponding section.
In the wavelength division optical amplification transmission system including the gain equalizers, the wavelength area is used as a signal wavelength band, and hence, as a matter of course, it is required that the gain equalizer be designed to have a loss wavelength characteristic the configuration of which is coincident with a gain wavelength characteristic of the equalization target module in a broad wavelength range. Even if a width of the wavelength range where the configurations are coincident is the same, a level of the optical signal after the gain equalization may differ in the case where their positions are different. For example, if a corresponding relationship between the gain wavelength characteristic of the equalization target module and the loss wavelength characteristic of the gain equalizer is as shown in FIG. 10(A), the optical signal after the gain equalization assumes a level corresponding to a gain Ga at a lower limit in a wavelength range where the configurations are coincident. By contrast, if the corresponding relationship between the gain wavelength characteristic of the equalization target module and the loss wavelength characteristic of the gain equalizer is as shown in FIG. 10(B), the width of the wavelength range usable as the signal wavelength band is the same as that shown in FIG. 10(A), however, the optical signal after the gain equalization assumes a level corresponding to a gain Gb larger than the gain Ga.
It is desirable that the level of the optical signal after the gain equalization be higher because the level is one of the parameters for determining a relay distance. Namely, it is desirable that an equalizer loss .DELTA.Lmax be small. Further, if the level of the optical signal after equalizing the gain is the same, as a matter of course, it is better to have a broader wavelength range usable as the signal wavelength band. Therefore, the gain equalizer is desired to have the loss wavelength characteristic the configuration of which is coincident with the gain wavelength characteristic over the entire wavelength range in which the gain of the equalization target module is equal to or greater than a given value.
Incidentally, it never happens that a manufactured gain equalizer has a loss wavelength characteristic the configuration of which is equal to the desired gain wavelength characteristic as shown in FIGS. 10(A) and 10(B) up to such a wavelength that the loss comes to "0". An actual loss wavelength characteristic of the gain equalizer is coincident in terms of its configuration (a relative value) with the desired gain wavelength characteristic with respect to only a portion in which the loss is above a certain value (Lo; a value called a excessive loss) as shown in FIG. 11. Namely, the level of the optical signal after the gain equalization corresponds to not the gain Ga at the lower limit wavelength in the signal wavelength band but to a gain Ga' smaller by the excessive loss Lo than Ga. Therefore, the gain equalizer is desired to have an excessive loss, Lo, as small as possible.
A conventional method of manufacturing (designing) the gain equalizer will hereinafter be described. Now, the normal optical filter (the one that can be manufactured at a high precision and assured in its reliability: e.g., a dielectric multilayer filter, an etalon filter and a Mach-Zehnder filter) has a gain wavelength characteristic with a high symmetry. It is therefore impossible to compensate an asymmetrical gain wavelength characteristic possessed by the equalization target module (the optical amplifier) in a broad wavelength if a gain equalizer is constructed of the single optical filter, and a situation as illustrated in FIG. 11 occurs.
Such being the case, it has been practiced that the gain equalizer having the loss wavelength characteristic assuming a configuration more approximate to the desired gain wavelength characteristic is obtained by combining a plurality of optical filters each having a loss wavelength characteristic with a periodicity.
For instance, according to a technique reported on p.4 onward of "First Optoelectric and Communications Conference Technical Digest, July 1996, Makuhari Messe", as shown in FIG. 12, the gain equalizer is structured by combining two Mach-Zehnder filters respectively Free Spectral Ranges FSRs (a difference between two adjacent loss maximum wavelengths) of 6 nm and 25 nm. Further, techniques of constructing gain equalizers by combining a plurality of Mach-Zehnder filters are also disclosed on pp. 982-983 of "ELECTRONICS LETTERS 9th June 1994 Vol.30 No.12" and Japanese Patent Laid-Open Publication No.4-147114 as well.
Disclosed further on p.578 of the report of preliminary articles in the Communications Society Convention by the Japanese Electronic Information Communications Association is a technique of constructing a gain equalizer by combining two etalon filters having loss wavelength characteristics corresponding to two sine waves with different periods, into which the gain wavelength characteristic of EDFA defined as an equalization target is developed by Fourier transform.
As known very well, the function of an arbitrary configuration can be expressed by a sum of some functions having periodicity. Accordingly, as in the prior art, even the gain equalizer assuming the loss wavelength characteristic in any configuration can be manufactured by combining the optical filters having the loss wavelength characteristics exhibiting the periodicity. Generally the excessive loss, however, becomes larger when a greater number of optical filters are combined. Therefore, a wavelength range, (i.e., a signal wavelength band) in which the gain can be equalized by the gain equalizer constructed by combining the optical filters having the loss wavelength characteristics exhibiting the periodicity, is on the order or 15 nm at the maximum.